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EQ6-R - alignment stars

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#1 the Elf

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Posted 21 September 2019 - 04:54 AM

Hello all,

 

hope this helps someone who is using the Skywatcher EQ6-R and doing manual star alignmnet (3-Star-alignment).

Put up my rig last night as always for AP. I do polar alignment with the polar scope. I spent quite some time to get it really centered and I do level the tripod precisely which results in a polar error in the range of 3 arcmins, reproducable. Stacking in PI always shows field rotation less than 0.01° over hours, so I'm sure I'm doing well here. To my surprise the mount controller told me I'm 30arcmin off the pole. Convinced this is not true I slewed to the target and it was way of the image center. So I went back to calibration and this time picked stars that seemed more suitable to me, that is not too low at the horizon, not to close to each other and a long slew between eastern and western half of the sky to make sure cone error is calculated correctly. This time it reported 3 arcmin polar error, as expected. The mount and tripod was not touched and I'm sure it does not move (Berlebach Planet small, spikes sunk in the lawn with my full body weight - not gonna tell you how much that is).

If I recall correctly the controllers standard setting is sorting alignment stars by brightness, sorting by alphabet is also possible. Conclusion: it suggests the brightest stars, no matter if the position is mathematically problematic to build the model. It seems like one should not blindly use the suggested stars, so from now on I will pick them according to the above rules.

If anyone experienced something similar or knows details what the controller is doing, feedback is apprechiated!

 

clear skies!

the Elf



#2 John Tucker

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Posted 21 September 2019 - 06:13 AM

I've heard something simlar to your comment about SynScan doing a poor job of picking alignment stars elsewhere, I don't remember where. 

 

Dunno if this is relevant to your post, but I'll toss it out there just in case.

 

I struggled with GoTo on my EQ6-R for the first year I had it.  I'd line up the first star, slew to the second and it wouldn't even be in the FOV of my 1200 mm (with focal reducer) SCT.  The third would be the same situation, sometimes even out of the FOV of my finder scope. Over time I just learned to plate solve, and started thinking of the GoTo as a tool to get my scope pointed into the right 10% or so of the sky. 

 

I'm not sophisticated enough to measure field rotation.  But I did find that after buying SharpCap and getting really obsessive about polar alignment, the GoTo performance improved dramatically.  These days I go out and the second and third star are almost always very close to the center, such that I could get nearly equivalent results with a one star alignment.  All night long I slew to things and they are not only in the FOV, but they are centered.  

 

I get some really crazy numbers off the handset after doing the alignment.  They often don't make sense in light of my results in SharpCap or drift aligning. I've even had it tell me that my polar alignment is so bad I should just stop and do the alignment over when I knew it was pretty good, and rebooting got rid of the message. I generally ignore the handset commentary about polar alignment.


Edited by John Tucker, 21 September 2019 - 06:39 AM.


#3 sg6

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Posted 21 September 2019 - 07:28 AM

How Synscan selects and offers alignment stars is very much a mystery. Ever get the idea they will work out Dark Matter before Synscan Alignment?

 

Don't think it can be brightest as one night I was asked to center Algieba in Leo when equally Regulus and Denebola were both easier and brighter. It never made sense.

 

The handset makes it harder as just one line at a time appears, scrolling up and down takes time. The Synscan App for Wifi use displays a page of about 20 at a time, so easier.

 

After meeting assorted alignments I am not sure what can be done about the Synscan approach to it, but it generally comes out as messy.

 

Going to have to power up the old PC as I have the Synscan alignment stars on that and the data. One item in the data is Month so that could come into operation. Although a good alignment star at 20:00 in the evening may not be a good one at 02:00 six hours later as it will have rotated by 90 degrees across the sky.

 

Having aligned a CPC 1100 I have the idea that maybe (just a maybe) alignment stars have to be seperated by some amount. One night I was offered Altair but not Vega or Deneb, they were not offered even when scrolling up/down the list. All I can think is that as Altair was an alignment star then Vega and Deneb were considered too close to be included - makes some sense. The unaware would otherwise just use the Summer Triangle and likely get poor goto's.



#4 David_Ritter

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Posted 21 September 2019 - 08:40 AM

I ran into this exact same situation regarding good-vs-bad stars as well. The hand controller software offers up what it thinks are the brightest stars, not the best stars. And selecting the brightest rather than the best can often lead to poor alignment. (whether they are actually the brightest is debatable too, but as far as I know, it's by magnitude in the hand controller, which is not necessarily the magnitude in the sky).

 

Synscan is pretty picky about which stars are best too. In the SynScan controller manual they suggest the following:

 

Rules for choosing alignment stars (two star alignment):

  The deviation in R.A. of the two alignment stars should not be too small or too close to 12 hours;

  The recommended deviation is between 3 hours and 9 hours.

  If there is cone error in the telescope-mount setup or if users are not sure about it, it is recommended to choose two alignment stars that are on the same side of the meridian.

 The absolute values of the two alignment stars’ declination should better deviate between 10 to 30 degrees.

 

( The short version:

  RA = 3 to 9 hours apart, avoid being near zero or 12hrs apart and both should be on the same side of the sky.

  DEC = 10 to 30 degrees apart, avoid the extremes at the poles

)

 

There are similar rules for 1 and 3 star alignments as well.

 

One really good resource to help find good alignment stars is here:

 

http://www.jimscosmo...lignment-stars/

 

Jim created a really good PDF map that shows all the alignment stars that SysScan uses. The map is a grid based on RA and DEC coordinates so it's really easy to find stars that meet the required criteria.


Edited by David_Ritter, 21 September 2019 - 08:46 AM.

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#5 the Elf

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Posted 21 September 2019 - 12:34 PM

Thank you very much for the feedback. Most of the time the alignment worked pretty well. Once the first star was centered, #2 and #3 were very close to the center of the field and at the right side, i.e. pusing the up and right key to move them to center. Looked like the mount intentionally did this, but the more I read the more I doubt this is the case. It was just the last two alignments that were so bad. Sometimes people write manuals and simply forget simple and clean statements like "The controller is not suggesting the best stars, the user has to pick a set that matches the rules." The list of stars is quite long. So far any star that I can identify and know it's name was in the list.

The AVX sorted stars by west and east. By pushing the right button it was possible to choose what side to start with. Any idea if SyncScan is doing the same thing? I will go out now and check what's in the list for star 1, 2 and 3 and if there is any system I understand.



#6 the Elf

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Posted 21 September 2019 - 04:37 PM

So, from what I understand the list is not sorted by east and west. It is the same list for all 3 stars. I used arcturus, dubhe and caph and polar error was 2 arcmins. The bubble nebula was about 5 arcmin of the image center. Hard to say if this is an error or if the nominal positon of the object is not where I place it in the image.




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